What causes printing errors in automatic flexo folder gluer and how to fix?

2025-09-30 16:15:13

Automatic Flexo Folder Gluers (AFFGs) play a pivotal role in delivering high-quality printed cartons for industries ranging from food and beverage to electronics. However, printing errors—such as misregistration, ink smudging, and uneven color—can compromise product aesthetics, lead to costly waste (often 5–15% of production runs), and delay order fulfillment. These errors rarely stem from a single cause; they typically arise from misalignments in machine components, incompatible material-ink combinations, or suboptimal operational settings. This article breaks down the most common printing errors in AFFGs, identifies their root causes, and provides step-by-step solutions to resolve and prevent them, ensuring consistent print quality and efficient production.

1. Misregistration: When Print Elements Are Out of Alignment

Misregistration is one of the most frequent and visible printing errors, occurring when design elements (e.g., logos, text, graphics) shift relative to each other or the carton blank’s edges. It is measured in millimeters (mm) and is deemed acceptable only if it falls below 0.1mm for high-quality packaging (e.g., cosmetic boxes) or 0.3mm for industrial cartons.

1.1 Causes of Misregistration

Web Transport System Misf alignment: The web (cardboard or substrate) must move at a consistent speed and direction through the AFFG. If conveyor belts are worn or misaligned, the web may drift sideways or experience speed fluctuations. For example, a conveyor belt stretched by 2% on one side can cause the web to shift 1–2mm per meter of travel. Additionally, worn nip rollers (with flat spots or uneven rubber) can slip against the web, creating speed mismatches between the web and print cylinders.

Print Cylinder Synchronization Issues: Each print cylinder (for different colors) must rotate at a speed perfectly matched to the web’s speed. If servo motors driving the cylinders are calibrated incorrectly, or if motor belts are loose or frayed, the cylinders may rotate too fast or slow. A 0.5% speed discrepancy between two cylinders can cause color misregistration of 0.5mm on a standard carton blank.

Material Dimensional Instability: Hygroscopic materials (e.g., SBS board, uncoated paperboard) absorb or release moisture based on environmental humidity, leading to expansion or contraction. For instance, SBS board exposed to 75% relative humidity (RH) can expand by 1.5% in width, causing the printed design to shift relative to the carton’s fold lines. Similarly, thin plastic films (e.g., PET) stretch under high web tension, distorting the print alignment.

Incorrect Plate Mounting: Flexographic plates must be mounted on the print cylinder with precise alignment (both circumferentially and laterally). If plates are mounted with air bubbles between the plate and cylinder, or if the mounting tape is unevenly applied, the plate may shift during printing. Even a 0.1mm misalignment during mounting can result in visible misregistration in the final print.

1.2 Solutions to Fix Misregistration

Calibrate the Web Transport System:

Inspect conveyor belts for wear or uneven tension—replace stretched belts and adjust tensioners to ensure uniform tension across the belt’s width.

Replace worn nip roller sleeves (aim for 60–70 Shore A hardness) and adjust nip pressure to 1–2 bar for paperboard or 2–3 bar for corrugated board, ensuring even contact with the web.

Use a laser alignment tool to check conveyor belt straightness; adjust belt guides to correct any sideways drift.

Synchronize Print Cylinders:

Use the AFFG’s HMI (Human-Machine Interface) to access the servo motor calibration menu. Run a synchronization test, which prints a registration mark pattern (e.g., crosshairs) on the web. Adjust the motor speed of each cylinder until the marks align within 0.05mm.

Inspect motor belts for fraying or slack—replace worn belts and tension them to the manufacturer’s specifications (typically 8–12 N·m torque for belt fasteners).

Stabilize Material Dimensions:

Pre-condition materials in a climate-controlled room (20–25°C, 40–60% RH) for 24 hours before processing to balance moisture content. Use a moisture meter to verify material moisture (target: 6–8% for paperboard).

Adjust web tension based on material type: reduce tension to 2–5 N/m for plastic films and lightweight paperboard to minimize stretching; increase to 5–10 N/m for corrugated board to maintain stability.

Re-Mount Print Plates Correctly:

Clean the print cylinder surface with isopropyl alcohol to remove dust or adhesive residue.

Apply double-sided mounting tape evenly across the cylinder, ensuring no air bubbles. Use a roller to press the tape firmly onto the cylinder.

Align the flexographic plate using registration marks on the plate and cylinder. Press the plate onto the tape from the center outward to eliminate air bubbles, then secure the plate edges with additional tape.

2. Ink Smudging: Blurred or Streaked Print

Ink smudging occurs when wet ink transfers to unintended areas of the substrate or machine components, resulting in blurred text, streaked graphics, or ink stains on the carton’s non-printed areas. It is particularly common with fast-drying inks (e.g., UV-curable) that are not fully cured, or slow-drying inks (e.g., water-based) that remain tacky during processing.

2.1 Causes of Ink Smudging

Inadequate Ink Drying/Curing: Water-based inks require sufficient time and airflow to evaporate moisture, while UV-curable inks need proper UV light intensity to polymerize. If the AFFG’s drying system (fans, heaters) is underperforming—e.g., a broken heater element reducing air temperature from 60°C to 35°C—water-based ink may remain wet. For UV inks, a dirty UV lamp (with dust or ink residue) can reduce light intensity by 30–50%, leaving ink uncured and tacky.

Excessive Ink Application: Anilox rollers with oversized cell volumes (e.g., 15 BCM instead of 8 BCM for paperboard) transfer too much ink to the print plate, leading to ink buildup on the substrate. Additionally, if the ink duct is set to deliver too much ink (e.g., 10% above the recommended flow rate), the excess ink cannot be fully absorbed or cured, resulting in smudging.

Web Contact with Machine Components: After printing, the wet web may come into contact with uncoated metal parts (e.g., guide rollers, folding plates) or dirty components. For example, a guide roller coated with dried ink can transfer the ink back onto the wet web, creating streaks. Similarly, if the web is not properly supported by idler rollers, it may sag and touch the machine frame, smudging the print.

Incompatible Ink-Substrate Combination: Non-porous substrates (e.g., plastic films, metalized board) do not absorb water-based ink, causing the ink to sit on the surface and remain wet. Using water-based ink on plastic films—instead of solvent-based or UV-curable ink—almost always results in smudging, as the ink cannot bond to the substrate’s surface.

2.2 Solutions to Fix Ink Smudging

Optimize Ink Drying/Curing:

For water-based inks: Clean or replace heater elements in the drying system to ensure air temperature reaches 50–60°C. Increase airflow by checking fan belts and cleaning air filters (clogged filters reduce airflow by 40%).

For UV-curable inks: Turn off the UV lamp, wipe the bulb with a lint-free cloth dampened in isopropyl alcohol, and replace the lamp if it has been in use for more than 2,000 hours (the typical lifespan of UV lamps). Use a UV light meter to verify intensity (target: 800–1,200 mW/cm²).

Reduce Ink Application:

Replace the anilox roller with one that has a cell volume matching the substrate: 3–5 BCM for smooth, porous substrates (e.g., SBS board), 8–12 BCM for rough substrates (e.g., recycled board).

Adjust the ink duct flow rate to the manufacturer’s recommended level—use a graduated cylinder to measure ink output over 1 minute, and reduce flow if it exceeds specifications.

Prevent Web Contact with Machine Components:

Install Teflon-coated or rubber-lined guide rollers (instead of bare metal) to prevent ink transfer. Clean all rollers and folding plates with solvent after each production run to remove dried ink.

Add additional idler rollers to support the web, ensuring a minimum distance of 10cm between the web and any non-essential machine parts. Adjust the web path using the HMI to eliminate sagging.

Use Compatible Ink-Substrate Pairs:

For non-porous substrates: Switch to solvent-based ink (for plastic films) or UV-curable ink (for metalized board). Test the ink on a small sample of the substrate to ensure adhesion and drying.

For porous substrates: Verify that the water-based ink has the correct viscosity (500–1,000 cP for paperboard) — thin the ink with distilled water if it is too thick, or add a thickener if it is too thin.

3. Uneven Color: Patchy or Faded Print

Uneven color manifests as areas of the print that are lighter (faded) or darker (patchy) than the intended shade. It undermines brand consistency—critical for consumer packaging—and often requires scrapping entire batches of cartons if the color variation exceeds industry standards (typically ±5% for spot colors).

3.1 Causes of Uneven Color

Anilox Roller Wear or Clogging: Over time, anilox roller cells become worn (reducing cell depth by 20–30% after 12 months of use) or clogged with dried ink, limiting ink transfer. A worn roller with shallow cells transfers less ink, causing faded areas; a clogged roller (with 10–15% of cells blocked) creates patchy print where ink cannot reach the substrate.

Inconsistent Print Pressure: The pressure between the print plate and the web must be uniform across the plate’s surface. If the print cylinder is misaligned (e.g., tilted by 0.5°), or if the plate has uneven thickness (due to poor manufacturing), pressure will be higher in some areas, transferring more ink (darker patches) and lower in others (faded areas). Additionally, worn plate mounting tape (with compressed sections) can create pressure variations.

Ink Viscosity Fluctuations: Ink viscosity (thickness) directly affects ink flow and transfer. If water-based ink absorbs moisture from the air (in high humidity), its viscosity decreases by 10–20%, making it runny and causing darker patches. Conversely, if solvent-based ink evaporates solvent (in low humidity), viscosity increases by 20–30%, making it thick and difficult to transfer, leading to faded areas.

Substrate Surface Irregularities: Rough or uneven substrates (e.g., uncoated recycled board, corrugated board with uneven flutes) have varying surface heights. The print plate cannot make consistent contact with high and low areas of the substrate—ink transfers well to high areas (darker) but poorly to low areas (faded). Similarly, a substrate with varying fiber density (e.g., recycled board with clumps of fibers) absorbs ink unevenly, creating patchy color.

3.2 Solutions to Fix Uneven Color

Restore or Replace Anilox Rollers:

Clean clogged rollers using an ultrasonic cleaner (fill with anilox cleaning solution, run for 30 minutes) to remove dried ink from cells. For worn rollers, measure cell depth with a laser depth gauge—if depth is reduced by more than 20%, re-engrave the roller or replace it.

Implement a weekly anilox cleaning schedule to prevent clogging: use a soft-bristle brush and cleaning solution to scrub the roller after each production run.

Ensure Uniform Print Pressure:

Use a pressure gauge to measure print pressure across the plate’s width. Adjust the print cylinder’s position (via the HMI’s alignment tool) to ensure pressure is 1–2 bar (paperboard) or 2–3 bar (corrugated board) uniformly.

Replace the plate mounting tape with a new roll (thickness tolerance ±0.02mm) to eliminate pressure variations. Use a roller to press the tape firmly onto the cylinder, ensuring no air bubbles or compressed sections.

Stabilize Ink Viscosity:

For water-based ink: Use a viscometer to measure viscosity hourly. If viscosity decreases (due to moisture absorption), add a thickener (1–2% by volume) to restore it to the target range. If viscosity increases (due to solvent evaporation), add distilled water (1–2% by volume).

For solvent-based ink: Seal the ink tank with a lid to minimize solvent evaporation. Add solvent (1–2% by volume) hourly to maintain viscosity, and use a viscometer to verify (target: 1,000–1,500 cP for plastic films).

Address Substrate Irregularities:

For rough substrates (e.g., recycled board): Increase print pressure by 10–15% to ensure the plate makes contact with low surface areas. Use a coarser anilox roller (100–150 LPI) to apply a thicker ink layer that fills surface irregularities.

For corrugated board: Use a “corrugated-specific” print plate with a thicker base (0.3mm instead of 0.2mm) to compensate for flute height variations. Adjust the web tension to 8–10 N/m to flatten the board slightly during printing.

4. Ink Adhesion Failure: Ink Peeling or Rubbing Off

Ink adhesion failure occurs when the ink does not bond to the substrate, causing it to peel off when rubbed (e.g., during carton folding or shipping) or when exposed to minimal pressure. This error is common with coated or non-porous substrates and can lead to customer complaints, as the printed design becomes unreadable.

4.1 Causes of Ink Adhesion Failure

Contaminated Substrate Surface: The substrate’s surface may have oils, dust, or coatings that prevent ink bonding. For example, plastic films are often treated with anti-static oils during manufacturing; if not cleaned, these oils create a barrier between the ink and film. Similarly, recycled board may have residual adhesives from previous packaging, which repel ink.

Incompatible Ink Chemistry: Ink must be formulated to bond with the substrate’s surface chemistry. For instance, using non-polar ink (e.g., for uncoated paperboard) on polar substrates (e.g., PET plastic, which has a high surface energy) results in poor adhesion, as the ink cannot “wet” the substrate. Similarly, UV-curable inks without a primer will not bond to metalized substrates, as the metal layer repels the ink.

Insufficient Surface Treatment: Non-porous substrates (e.g., plastic, metalized board) require surface treatment to increase their surface energy, allowing ink to adhere. If a corona treater (used to ionize the substrate surface) is not functioning—e.g., a broken electrode or low power output—the substrate’s surface energy remains too low (below 38 dynes/cm for plastic films), and ink cannot bond.

Under-Cured Ink: For UV-curable or two-part inks, incomplete curing leaves the ink’s chemical structure unstable, reducing its adhesion to the substrate. A UV lamp that is too old (output <600 mW/cm²) or a two-part ink mixed with the wrong catalyst ratio (e.g., 1:10 instead of 1:5) will not fully cure, resulting in weak adhesion.